Conventional heaters, such as are utilized in aircraft sensors, or even for laboratory testing, use a variety of materials to produce heating elements having relatively high operating temperatures. However, these materials suffer from the detrimental effects of contamination, ionic migration, sublimation, oxidation and substantial decrease in mechanical strength with increased operating temperatures. Current electrical heating elements are thus limited to an operating envelope in the range of less than 650° C. (1200° F.) to ensure long term, stable output. Higher temperature heating devices may operate to temperatures up to 850° C. (1562° F.), but are either limited to specific environmental conditions (such as for instance: a vacuum environment, an inert gas environment, or a hydrogen atmosphere) and/or must be limited to short term operation to prevent premature failure. This temperature operating range has limited the application of these heating devices in for example, hostile, high temperature applications such as those commonly encountered in the aerospace, petroleum, glass industries, and laboratory testing applications.
Resistive electrical heating devices are useful for providing ambient heating. They are inexpensive and relatively simple to operate, where upon connection to a power source; the electrical resistance heating device will produce a repeatable heat output proportional to the applied electrical power. This provides a simple and inexpensive heating system. However, prior art electrical resistance type heaters have suffered from the problem of being limited to a fairly low melting temperature and, accordingly, have not been useable to provide substantial heating, such as in systems requiring heating up to 1500° C. (2730° F.). Not only have external thermal fields been a problem, resistive electric heaters have also typically, been unusable in environments where they are exposed to mechanical stress. In addition, electrical resistive heaters also suffer from the detrimental effects associated with the transmission of relatively high current levels.
Platinum is known to have a relatively high melting point and as such, may be desirable for use as a heating element. Platinum provides a number of advantages, such as being chemically stable and having highly repeatable heat output with applied electrical power. Other high melting, noble metals such as rhodium (Rh), palladium (Pd), iridium (Ir) as well as precious metals such as gold (Au) and silver (Ag), and alloys thereof are known. However, it should be noted that these materials do not offer the mix of strength, oxidation resistance, rupture strength at elevated temperature, resistivity, alpha, or oxide stability as Pt and Pt/Rh based materials. This can be critical in highly sensitive experimentations, such as is required in laboratory experimentation.
Some of the characteristics of platinum can be improved by the alloy hardening method of adding a metal to the platinum base, followed by a heat treatment. However, problems can occur after alloying. For example, when a high concentration of any alloying element is added to the platinum base, the electrical properties of the resulting platinum limb become inferior; at the same time the hardening phase will partially or totally dissolve into the base at high temperatures, thus the effects of the hardening action are disadvantageously reduced.
Dispersing oxides of transition metals or rare earth metals within noble or precious metals is an example of a method of creating a resistance material with the desired extended temperature properties. For instance, dispersion hardened platinum materials (Pt DPH, Pt-10% Rh DPH, Pt-5% Au DPH) are useful materials because they achieve very high stress rupture strengths and thus permit greatly increased application temperatures than the comparable conventional alloys and are rugged.
Dispersion hardening (DPH) creates a new class of metal materials having resistance to thermal stress and corrosion resistance that is even greater than that of pure platinum and the solid solution hardened platinum alloys. When operational life, high temperature resistance, corrosion resistance and form stability are important, a heater may be manufactured of DPH platinum and can be used at temperatures close to the melting point of platinum.
Dispersion hardened materials contain finely distributed transition element oxide particles which suppress grain growth and recrystallization even at the highest temperatures and also hinder both the movement of dislocations and sliding at the grain boundaries. The improved high temperature strength and the associated fine grain stability offer considerable advantages. The article, “Platinum: Platinum-Rhodium Thermocouple Wire: Improved Thermal Stability on Yttrium Addition Platinum” By Baoyuan Wu and Ge Liu, Platinum Metals Rev., 1997, 41, (2), 81-85 (“the Wu article”) is incorporated by reference. The Wu article discloses a process of dispersion hardening platinum for a platinum; platinum-rhodium thermocouple wire which incorporates traces of yttrium in the platinum limb.
As described in the Wu article, the addition of traces of yttrium to platinum as a dispersion phase markedly increases the tensile strength of the platinum at high temperature, prolongs the services life and improves the thermal stability. Yttrium addition prevents the growth in the grain size and helps retain the stable fine grain structure, as the dispersed particles of high melting point resist movements of dislocations thereby maintaining rupture strength at elevated temperature without a loss of ductility.
In order to harden metals, the movement of the dislocations needs to be restricted either by the production of internal stress or by putting particles in the path of the dislocation. After the melting and processing, the majority of the trace yttrium (in the dispersion phase of the platinum) becomes yttrium oxide, which has a much higher melting point than platinum. When the temperature is near the melting point, dispersion hardened particles fix the dislocation, thus hardening the platinum and increasing its strength.
At the same time the grain structure becomes stable after dispersion hardening and there is also microstructural hardening. The dispersed particles affect the recrystallization dynamics, inhibit rearrangement of the dislocations on the grain boundaries and prevent the movement of the grain boundaries. Therefore, this dispersion hardened platinum possesses a stable fine grain structure at high temperature.
This patent outlines an electrical resistance heating element that is capable of operating in the range of 1700° C. (3092° F.).
Accordingly, it is an object of the present invention to provide an electric heater exhibiting high mechanical hardness for protection of the heating element and/or conductors connected thereto.
Accordingly, it is another object of the present invention to provide an extended temperature range electrical resistance heater with enhanced high temperature operating characteristics and long term, stable output and minimum drift.
Yet another object of the present invention is to provide a method for the production of a cost effective, high reliability, stable resistance heater with an operating range of up to 1700° C. (3092° F.) in hostile environments.